This is kind of... Well, good news, I suppose? It depends on where you allegiances lie, but it seems like Ubuntu is warming up to the idea of using Qt to develop applications. It's no secret that Qt is a far more advanced development framework than Gtk+, so it only makes sense for Ubuntu - a GNOME/Gtk+ distribution - is looking at it.

The benefits I found (I started using GTK then switched to Qt), in order of significance.

* Signals/slots. They may be slightly hacky, but it is hidden well and they really are the easiest way to code UIs. Much better than callbacks (wxWidgets, GTK) or message passing (MFC).

* Documentation. The Qt documentation is one of the best of any project I've used. It's very comprehensive, and tells you exactly what you need to know. It's like the opposite of Android documentation.
It's comparable, but slightly better than the MySQL, or MSDN docs.

* It's real C++. It isn't a hacked up a "C++ in C's clothing" using endless macros and casts like GTK is.

* More comprehensive. It includes way more useful stuff than just low-level UI things. Particularly QtXML, QtWebKit, QGraphicsView, and the network stuff.

* Qt Creator (ok, this didn't exist when I started using Qt, but still). It rocks. A sample of the many awesome features: 1. It supports smart tabs! 2. Code-completion actually works *cough* KDevelop, Anjuta *cough*. 3. Ctrl-click anything to go to its definition. Very useful.

* Performance on Windows. It doesn't suck. And it looks native, unlike GTK.

There's simply no reason to use GTK over Qt today. The only real reason was the licence, but that is resolved.